1. Field of the Invention
This invention relates to detection methods and apparatus and in particular it concerns novel sensor elements, novel methods for making such elements and novel methods and apparatus for detecting such elements.
2. Description of the Prior Art
The present invention is particularly useful in the electronic article surveillance industry, although as will be seen hereinafter, certain aspects of the invention may have application to other industries.
In the article surveillance industry, articles of merchandise, e.g. books, clothing, etc., are protected from theft or other unauthorized removal from a protected area by securing to the articles a target element and providing a target monitor at each exit from the protected area.
One particularly successful article surveillance system is shown and described in detail in U.S. Pat. No. 4,623,877 in the name of Pierre F. Buckens and assigned to the assignee of the present invention.
As shown and described in that patent, elongated target strips of magnetically soft, i.e. easily saturable, low coercivity material are attached to articles to be protected. A transmitter and a receiver are provided with antennas located at the exit from a protected area. The transmitter generates a continuous alternating magnetic field at the exit and when an article with a target strip attached is carried through the exit, the strip is magnetically saturated successively in opposite directions by the alternating magnetic field and thereby produces distinctive disturbances in the field. The thus disturbed field is received by the receiver. The receiver in turn produces corresponding electrical signals and processes them to detect those corresponding to the particular distinctive disturbances produced by the target strips and to actuate an alarm in response to the selected signals.
It is important that the target strips have very distinctive magnetic characteristics so that the disturbances they produce can be distinguished from the disturbances produced by ordinary magnetic objects. In general, targets for such systems should have very low magnetic coercivity, e.g. less than 0.1 oersted; they should be easily magnetically saturable; and they should have a magnetic hysteresis loop which is generally rectangular in shape. Thin elongated strips of a crystalline material, such a Permalloy.RTM. or an amorphous material, such as Metglas.RTM. have been used in the past as detectable targets.
Various techniques have been used in the past to improve the magnetic characteristics of these target strips to increase the distinctiveness of the disturbances they produce on an incident alternating magnetic field. One such technique, which is described in U.S. Pat. No. 4,660,025, involves maintaining a mechanical stress on the material to increase its Barkhausen characteristics. Another technique, which is described in U.S. Pat. No. 4,326,198, involves applying a continuous magnetic bias to the target strip so that it will produce a preponderance of even harmonics of the incident alternating magnetic field frequency.
It has been observed, although only as a magnetic phenomenon in connection with certain hard magnetic materials and not as a technique having anything to do with easily saturable magnetic materials or sensing devices, that extremely small particles, e.g. 100 to 1000 angstrom units in transverse dimension, of ferromagnetic iron, cobalt or nickel which share a common interface and are magnetically coupled with an antiferromagnetic material of iron oxide, nickel oxide or cobalt oxide will exhibit a shifted hysteresis characteristic with a higher remanence when magnetically polarized in one direction than when magnetically polarized in the opposite direction. Material of this type is described in U.S. Pat. No. 2,988,466.
Another problem associated with electromagnetic sensor devices in the electronic article surveillance industry is that of deactivating the devices so that the articles to which they are attached can be taken from the protected area without activating the alarm when removal has been authorized, for example upon purchase or proper checkout of an article of merchandise. In the past, the devices were provided with spaced apart elements or a continuous strip of a high coercive force magnetic material which, when magnetized, would effectively prevent the device from responding to the alternating magnetic field generated at the exit. The addition of these elements or strips was costly, both in terms of material and assembly; and they substantially increased the physical size of the sensor device. Also, although continuous strip type deactivatable sensor devices were easier to assemble than those with spaced apart elements, the former required actual physical contact with a deactivating magnet in order to effect deactivation; and thus they were less convenient to use.